Method and apparatus for optimizing current consumption of amplifiers with power control

ABSTRACT

Apparatus and method for reducing the current consumption and increasing the efficiency of an RF power amplifier (PA), according to which the load, connected to the output stage of the PA, tuned dynamically or statically for each level of the desired output power. By doing so, the output impedance of the output stage is essentially matched, such that the dynamic RF load line has a slope that corresponds to the impedance required to provide this level. Whenever a smaller output power is desired in response to reduction in the input signal to the amplifier, the load is further tuned, such that the dynamic or static RF load line has a slope that causes the power amplifier to essentially remain in saturation at the smaller output power.

CLAIM OF PRIORITY UNDER 35 U.S.C. §120

The present application for patent is a continuation of patentapplication Ser. No. 12/094,639 entitled “METHOD AND APPARATUS FOROPTIMIZING CURRENT CONSUMPTION OF AMPLIFIERS WITH POWER CONTROL” filedMay 22, 2008, allowed, which claims priority to International PatentApplication No. PCT/IL2006/001372, filed Nov. 28, 2006, which claimspriority to Provisional Application No. 60/741,550, filed Nov. 28, 2005,and assigned to the assignee hereof and hereby expressly incorporated byreference herein.

FIELD OF THE INVENTION

The present invention relates to the field of high efficiency poweramplifiers. More particularly, the invention relates to a matchingtechnique for reducing current consumption of RF power amplifiers whenno change in output power is required.

BACKGROUND OF THE INVENTION

RF amplifiers normally achieve their maximum efficiency while being atsaturation level. In many wireless applications, controlling outputpower as a function of receiving level is required. Withdrawal fromsaturation level will reduce efficiency. According to conventionalsolutions, the power stage is designed to handle the highest expectedpower, leading to lower efficiency at lower output power levels due tohigh quiescent currents.

One of the conventional ways to eliminate this problem is to control theDC supply of the power amplifier as a function of the power level. Thissolution is described for example, in U.S. Pat. No. 6,437,641, whichdiscloses circuitry for dynamically enhancing the operating voltage ofan RF amplifier. The resulting input drive to the PA is increased,thereby pushing its output well into saturation and handles the signalclipping by instantaneously enhancing the power supply voltage. However,additional active circuitry is required for such enhancement.

The efficiency of non-linear power amplifiers (which do not operateunder class A conditions) is usually reduced with output powerreduction. In many cases, especially where increased linearity is notneeded, this behavior might be avoided. Other conventional ways ofcontrolling efficiency are: using variable power supplies or enhanceddynamic loading techniques, such as a Linear Nonlinear Component (LINC)amplifier (that amplifies a signal which has amplitude variations thatare generated by two or more signals, which vary only in their relativephases. The vector sum of the two signals can represent any amplitude)or an amplifier that operates in Doherty configuration (described forexample, in U.S. Pat. No. 2,210,028).

Still, the methods described above have not provided satisfactorysolutions to the problem of improving the efficiency of power amplifiersoperated under large peak-to-average ratio input signals.

It is an object of the present invention to provide a method forefficiently controlling the current consumption of the PA for each levelof output transmitting power.

It is another object of the present invention to provide a method forefficiently controlling the impedance at the output of the RF path forincreasing the RF large signal gain of the amplifier without increasingthe current consumption of the PA.

It is another object of the present invention to provide circuitry forefficiently controlling the PA current consumption, gain and outputpower independently.

Other objects and advantages of the invention will become apparent asthe description proceeds.

SUMMARY OF THE INVENTION

The present invention is directed to a method for reducing the currentconsumption and increasing the efficiency of an RF power amplifier (PA),according to which the load, connected to the output stage of the PA,tuned dynamically or statically for each level of the desired outputpower. By doing so, the output impedance of the output stage isessentially matched, such that the dynamic RF load line has a slope thatcorresponds to the impedance required to provide this level. Whenever asmaller output power is desired in response to reduction in the inputsignal to the amplifier, the load is further tuned, such that thedynamic or static RF load line has a slope that causes the poweramplifier to essentially remain in saturation at the smaller outputpower.

Whenever the RF power amplifier is a part of a MIMO system that isrequired to provide a desired total output power level and consisting ofa plurality of PAs, the efficiency of the MIMO system is increased bydetermining an individual output power level for each PA, which is aportion of the total output power level. The load, connected to theoutput stage of each PA is dynamically or statically tuned toessentially match the output impedance of its output stage, such thatits dynamic or static RF load line has a slope that corresponds to theimpedance required to provide the portion. Whenever a smaller outputpower is desired in response to reduction in the input signal to MIMOsystem, a reduced portion for each PA is determined. The load is furthertuned, such that the dynamic RF load line has a slope that causes eachPA to essentially remain in saturation while providing the reducedportion.

Preferably, the PA is operated under Class B, Class AB or Class C.

The present invention is directed to an apparatus for reducing thecurrent consumption and increasing the efficiency of an RF poweramplifier (PA), that comprises a matching circuitry for dynamically orstatically tuning the load, connected to the output stage of the PA, toessentially match the output impedance of the output stage for eachlevel of the desired output power, such that the dynamic RF load linehas a slope that causes the power amplifier to essentially remain insaturation at the level.

The apparatus may be used for matching the output of an RF poweramplifier that is a part of a MIMO system and that is operated underClass B, Class AB or Class C.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other characteristics and advantages of the invention willbe better understood through the following illustrative andnon-limitative detailed description of preferred embodiments thereof,with reference to the appended drawings, wherein:

FIG. 1 schematically illustrates tuned load lines, according to apreferred embodiment of the invention;

FIG. 2 illustrates a 50Ω to 75Ω matching circuit; and

FIG. 3 illustrates the simulation results on a Smith Chart, for a 75Ωload impedance.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention provides a method and apparatus for achieving thesame efficiency for various values of output power by using differentvariable loading technique and re-matching of power amplifiers in orderto reduce their current consumption. An additional advantage obtained byusing the method proposed by the present invention is achieving higherpower gain with the same power amplifier transistors.

Additional applications of the proposed method are in wirelesstransmitters with power control and adaptable Multiple Input-MultipleOutput (MIMO) arrays. Changing the load line in a wireless transmittercan be done once per amplifier or once per output power. The efficiencyof a MIMO array can also be significantly improved using the proposedmethod by controlling each path independently.

The method proposed by the present invention allows reduction of currentconsumption of a power amplifier (PA). This method involves changing thematching circuit of the RF power amplifier, in order to reduce thecurrent and enhance the power gain of the amplifier on the expense ofamplifier output power saturation point.

Two kinds of RF matching techniques are commonly used for poweramplifiers. Maximal Power Gain is achieved while using conjugatematching circuit. Higher output 1 dB compression, but lower power gainis achieved while using a power-matching circuit. In cases where thecurrent and voltage limitations of the power transistors are dominant,power matching is usually used. In many situations, power matchingyields 2 dB higher compression point, as described, for example, inCripps, C. Steve, “RF Power Amplifiers for Wireless Communications”,Artech House, 1999. This result is somewhat dependent on the transistorbeing matched.

FIG. 1 schematically illustrates tuned load lines, according to apreferred embodiment of the invention. Load line A is for a conventionalclass B Power amplifier. When operating with this load line, reducingoutput power can be achieved by reducing the input power. However, thecurrent consumption will only be slightly reduced, and the efficiency isdegraded. In contradiction, load line B is obtained, according to theinvention, after tuning the amplifier for lower output power. It can benoticed that the length of load line B is shorter than the length ofload line A. This is the reason for the lower power handling capabilityof the amplifier. The graph of FIG. 1 also clearly shows how reducedcurrent mode is achieved. The efficiency in this case is not degraded.The method proposed herein allows adaptively changing the load line,while matching the optimal load line for each output power level.

High efficiency amplifier classes of operation with reduced conductionangle such as class AB, Class B and Class C configurations are verywidely used. Extensive analysis of the expected efficiency of thesemodes can be found for example in Cripps, as well. Although the analysisbelow is an implementation of the proposed method for class B, similarimplementations for any other conducting angle and power enhancement canused with the method proposed by the present invention.

EXAMPLES Example 1 Adaptive Load Line Changing for Power ControlledSystem

Most of the advanced power amplifiers have internal pre-matching and noexternal matching is required while using a 50Ω load. For this example,it is assumed that the amplifier used is pre-matched to a 50Ω load.Increasing the output load line impedance can be done using classicalmatching techniques. FIG. 2 hereinbelow is an example for such matchingcircuit and FIG. 3 hereinbelow is its simulation results for 75Ω loadimpedance.

The load line curve can be adaptively changed by simply tuning theparallel capacitors shown in the matching circuit FIG. 2. Load lines inthe range of 50-85Ω have been tested.

As an example, a design of a power controlled power amplifier for aGlobal System for Mobile Communications (GSM) mobile phone workingaccording to the 3GPP GSM standard TS 45.005 is illustrated. Control ofnominal output power is done in 2 dB steps. The maximum output powerlevels for handset mobile stations class 4 GSM is +33 dBm (for 850/900MHz band) and for class 1 Digital Cellular Service (DCS) and PersonalCommunications Service (PCS) is +30 dBm. The dynamic range for powercontrol is 28 dB for the 850/950 MHz band and 30 dB for the 1800/1900MHz band (see for example, “Output Control loop design for GSM Mobilephone” by Alex Gil-Garcia, Agilent technologies Inc. Santa Clara,Calif., U.S.A.). The required mobile output signal strength isdetermined by the distance between the mobile device and the basestation and, by environmental conditions. 2 dB front end and mismatchlosses are expected, leading to a higher PA output power level. Thepower amplifier efficiency for such an amplifier may be as high as 57%at the maximum output level (such as PA RF3146, which is a Power AmpModule manufactured by RFMD Greensboro, N.C., U.S.A.), but reduced to 2%at the lower power level. This situation can be eliminated using themethod proposed by the present invention. This is done by dynamicallychanging the load line, keeping the highest efficiency for the firstfour steps, and keeping this load line for the rest of the power levels.The efficiency at lower power level is also increased and we will get 5%for the lowest power level. This might reduce the current consumptionand increase the batteries lifetime by a factor of two.

Such implementation of dynamically changing the output load can beimplemented in various applications including: cellular phones from allsystems and frequencies, WiMax applications, WiFi applications, and allapplications where output power needs to be changed dynamically.

Example 2 Adaptable MIMO Arrays

For a specific MIMO system with up to four amplifier paths, the totalpower has been designed to stay constant for all operational modes. Thatmeans that for the case where only one amplifier operates, it shouldwork at maximum power. If more paths are operating, the power of each ofthem is reduced by a factor of the number of paths. For optimaloperation, the first amplifier was designed to work at maximal powerlevel, with adaptive load line control, similar to the previous example.The others were designed to work with lower power level. The systemefficiency remains high for all system modes.

For a MIMO W-LAN system working according to the IEEE standard 802.11nwith up to four transmit paths, the total output power is limited to 100mW (20 dBm), due to the to total output power limitations for systemsoperating in the un-licensed frequency bands. If for example, we havefour Power Amplifiers (PAs) that comply with the standard maskrequirements at maximum output power of 20 dBm for each PA and withefficiency of 20% per PA at that output power, when the system isoperated in the mode where only one PA is active, the load line of thatPA is tuned such that the output power is 20 dBm according to thestandard's limitations and the current consumption yields an efficiencyof 20%.

When the system is operated in a mode where two PAs are activesimultaneously, the load line of each PA is changed, such that theoutput power of each PA is tuned to 17 dBm, thereby producing a totaloutput power of 20 dBm from the system. When the system is operated in amode where three PAs are active simultaneously, the load line of each PAis changed such that the output power of each PA is tuned to 15 dBm,thereby producing a total output power of 20 dBm from the system. Whenthe system is operated in a mode where the four PAs are activesimultaneously, the load line of each PA is changed such that the outputpower of each PA is tuned to 14 dBm, thereby producing a total outputpower of 20 dBm from the system. The change in Load line of each PAallows keeping the total system efficiency at 20% for all cases.Practically small degradation is expected due to quiescent current ofthe PAs, which cannot be reduced linearly with load line changes. Thisway, the current drawn by the power amplifier while operating at variousoutput power levels is reduced, while maintaining a better RF powergain, compared to the compatible class of operation.

The above examples and description have of course been provided only forthe purpose of illustration, and are not intended to limit the inventionin any way. As will be appreciated by the skilled person, the inventioncan be carried out in a great variety of ways, employing more than onetechnique from those described above, all without exceeding the scope ofthe invention.

The invention claimed is:
 1. A method, comprising: tuning a loadconnected to an output stage of a power amplifier to impedance match anoutput impedance of the output stage, the tuning based on maintaining aslope of an RF load line of the power amplifier to correspond to animpedance providing a higher desired output power level while the poweramplifier operates in saturation for a plurality of higher output powersteps and changing the slope of the RF load line of the power amplifierto correspond to an impedance providing a lower desired output powerlevel to maintain the power amplifier in saturation for a plurality oflower output power steps; and adjusting a level of power enhancement tothe power amplifier to match the tuned load at the higher and lowerdesired output power levels.
 2. The method of claim 1, furthercomprising whenever a lower output power level is desired in response toreduction in an input signal to the power amplifier, further tuning theload, such that the RF load line has a slope that causes the poweramplifier to remain in saturation at the lower desired output powerlevel.
 3. The method of claim 2, further comprising further adjustingthe level of enhancement to match the lower desired output power level.4. The method of claim 1, wherein whenever the RF power amplifier is apart of a MIMO system that is required to provide a desired total outputpower level and consisting of a plurality of similar power amplifiers,the method further comprising increasing the efficiency of said MIMOsystem including for each power amplifier: determining an individualoutput power level as a portion of a total output power level; andstatically tuning the load connected to an output stage of a poweramplifier to impedance match an output impedance of the output stage,the tuning based on maintaining a slope of the RF load line of the poweramplifier to correspond to an impedance providing the portion while thepower amplifier operates in saturation.
 5. The method of claim 4,further comprising: determining a reduced portion for each of the poweramplifiers whenever a lower output power level is desired in response toreduction in the input signal to the MIMO system; and further tuning theload to maintain a slope of the RF load line of the power amplifiers tocorrespond to an impedance providing the reduced portion while the poweramplifiers operate in saturation.
 6. The method of claim 1, wherein thepower amplifier operates in one of Class B, Class AB or Class C modes.7. An apparatus, comprising: matching circuitry configured to tune aload connected to an output stage of a power amplifier to impedancematch an output impedance of the output stage, the tuning based onmaintaining a slope of an RF load line of the power amplifier tocorrespond to an impedance providing a higher desired output power levelwhile the power amplifier operates in saturation for a plurality ofhigher output power steps and changing the slope of the RF load line ofthe power amplifier to correspond to an impedance providing a lowerdesired output power level to maintain the power amplifier in saturationfor a plurality of lower output power steps; and circuitry configured toadjust a level of power enhancement to the power amplifier to match thetuned load at the higher and lower desired output power levels.
 8. Theapparatus of claim 7, wherein whenever a lower output power level isdesired in response to reduction in an input signal to the poweramplifier, the matching circuit further configured to tune the load,such that the RF load line has a slope that causes the power amplifierto remain in saturation at the lower desired output power level.
 9. Theapparatus of claim 8, wherein the circuitry is further configured toadjust the level of enhancement to match the lower desired output powerlevel.
 10. The apparatus of claim 7, wherein whenever the RF poweramplifier is a part of a MIMO system that is required to provide adesired total output power level and consisting of a plurality ofsimilar power amplifiers, the apparatus further configured to increasethe efficiency of said MIMO system including for each power amplifier,the apparatus further configured to: determine an individual outputpower level as a portion of a total output power level; and staticallytune the load connected to an output stage of a power amplifier toimpedance match an output impedance of the output stage, the tuningbased on maintaining a slope of the RF load line of the power amplifierto correspond to an impedance providing the portion while the poweramplifier operates in saturation.
 11. The apparatus of claim 7, whereinthe apparatus is part of a MIMO system.
 12. The apparatus of claim 7,wherein the power amplifier of the apparatus operates in one of Class B,Class AB or Class C modes.
 13. A method, comprising: tuning a load lineof a power amplifier that uses power enhancement to maintain a slopethat corresponds to an impedance required to provide a desired totalpower from the power amplifier while in saturation for a plurality ofhigher output power steps and to change the slope to correspond to animpedance providing a higher desired output power level to maintain thepower amplifier in saturation for a plurality of lower output powersteps; and adjusting a level of the power enhancement to the poweramplifier to match a tuned load at the higher desired output powerlevel.
 14. An apparatus, comprising: matching circuitry configured totune a load line of a power amplifier that uses power enhancement tomaintain a slope that corresponds to an impedance required to provide adesired total power from the power amplifier while in saturation for aplurality of higher output power steps and to change the slope tocorrespond to an impedance providing a higher desired output power levelto maintain the power amplifier in saturation for a plurality of loweroutput power steps; and circuitry configured to adjust a level of thepower enhancement to the power amplifier to match a tuned load at thehigher desired output power level.